Process for the liquefaction of natural gas



June 7, 1966 s. B. .JACKSON ET AL 3,254,495

PROCESS FOR THE LIQUEFACTION OF NATURAL GAS Filed June 10, 1963 2 Sheets-Sheet 1 June 7, 1966 s. B. JAcKsoN ETAL 3,254,495

PROCESS FOR THE LIQUEFATION OF NATURAL GAS INVENTORS.

33% f Ww United States Patent 3,254,495 Pnocnss non THE LrogllzgacrroN or NATURAL -Steven B. Jackson, Fullerton, and Donald E. Wheeler, El Monte, Calif., assignors to The Fluor Corporation, Ltd., Los Angeles, Calif., a corporation of California Filed .lune 10, 1963, Ser. No. 286,662 6 Claims. (Cl. 6212) This invention has to do with an improved process for the liquefaction of natural 4gas for the major commerical purpose of achieving the advantages of greatly lowered volumes for storage and transportation that result from maintenance of a desired product composition of the gas in liquefied condition.

Generally considered, the invention is directed to the liquefaction of natural gases characterizable as normally existing in the vapor state at atmospheric pressure and temperatures above 40 F., and which variably may contain normal paraiinic hydrocarbons (such as methane, ethane, propane, butanes, etc.), cycloparainic hydrocarbons (such as cyclopropane, cyclobutane, etc.), loleinic hydrocarbons (such as ethylene, propylene, etc.), acetylenes (such as ethyne, propyne, etc.), dioleiins (such as 1,3-butadiene, isoprene, etc.), aromatic hydrocarbons (such as benzene, toluene, etc.), and possibly nonhydrocarbons (such as nitrogen, helium, argon, hydrogen, carbon dioxide, hydrogen sulfide, carbonyl sulfide, water Vapor, etc.). Natural gases treatable in accordance with the invention may contain any of various combinations of the stated components, and the present process is so conceived as to be capable of eliminating any undesirable components and render the remainder as a liquid suitable for storage, transportation or any other need for natural gas inliquid state.

In actual practice, the invention will be employed primarily for the liquefaction of natural gases which are predominately methane, with moisture content up lto saturation, and which may contain separable components such as nitrogen, helium and the like, which do not undergo liquefaction and require separation from the mainly methane liquefied product going to storage or other disposal.

Our general object is to provide in a process which utilizes a unique combination of steps and conditions for the economic and eflicient liquefaction of a natural gas,4 and presenting advantages over processes heretofore employed, such as minimized compressor horsepower consumption, minimized equipment requirement, and capaicty for simplified operation and control. The process and equipment employed have easy adapability to such conditions as changing gas compositions, capacity and iinal product contr-o1, all in a manner permitting of economized initial investment, operational and maintenance expenses for given production.

Liquefaction of moisture-containing natural gas must take into account such several requirements of considerations as moisture removal to a degree approaching completeness, separation of unwanted components such as heavier hydrocarbons and fixed gases, as well as efficient and economical refrigeration to the linal stage o liquefied product recovery.

First, with respect to dehydration of the gas, the invention has as an important object to provide a novel system of moisture removal to a degree corresponding to or approaching complete dehydration, by the employment of one or more antecedent dehydrating stages, followed by deliberate acceptance of moisture freeze out in lthe system; but in a manner permitting of alternation which obviates excessive or interfering ice or other solids accumulations. More particularly, the invention contemplates subjecting the gas initially to partial dehyice dration at one or more locations along the gas stream flow, in conjunction with successive cooling or refrigeration stages, following which the gas is further cooled by alternating its passage through two or more cooling eX- changers, all in a manner such that solids freezing out is deliberately accepted in an on-stream exchanger, but

continuity of service is assured by alternating -to -a de-iced exchanger when excessive accumulations occur in the ori-stream exchanger.

Initial dehydration is efficiently accomplished preferably by introducing to the gas stream a liquid desiccant, which may be any of various known liquid absorbents capable of substantial moisture acceptance of dissolu- Ition, and which may be easily regenerated, as by simple heating. A-s illustrative, we find ethylene glycol to be desirable as `a liquid moisture absorbent, by reason of its absorption capacity, low corrosion tendencies and easy regenerability. As will appear, we preferably provide for successive interval or stage injection of the glycol into the gas stream, accompanied by corresponding withdrawals and regeneration in a common heating zone, Ifrom which the lean glycol may be taken for separate stream return to the gas stream undergoing stage coolmg. I

The invention further contemplates final stage cooling of the liquefied gas beyond the alternate exchangers, and after removal of unwanted vaporized or gaseous componen-ts, by a system of auto-refrigeration by Joule- Thompson eitect resulting from ash vaporization of a portion of the liquefied stream, whereby the temperature of the liquid residue is lowered suiiciently for storage or transportation purposes. In conjunction with the final ash vaporization stage, provision is made for recovery of the vapors produced, by subjecting them lto compression and return to the main gas stream undergoing liquefaction, all as will later appear.

The invention has various additional features and objects, all of which will be more readily understood from the following description of a typical embodiment of the invention as illustrated by the accompanying drawings in flow sheet form and wherein:

FIG. 1 is the flow sheet through the high level and low level refrigeration stages; and

FIG. 2 is a continuance of FIG. 1, illustrating the autorefrigeration and storage stage.

The system may be assumed to be supplied with feed gas through line 10 composed predominately of methane, and typically about methane, 2% nitrogen, 11% ethane, 5% propane, 3% butanes and heavier, the gas being saturated with water vapor. (The percentages are by volume.) The feed gas may further be assumed typically to enter the system at about 70 F. and 520 p.s. i.a., which is an illustrative pressure at which the desired product components of the gas will undergo liquefaction in the system. It may be further assumed that the feed gas is essentially free from hydrogen sulfide and other cornpounds that might present corrosion or freezing-out problems in the system. Such pre-conditioning of the gas may be accomplished by suitable known treating processes. Ordinarily, however, as fed to the system, the gas may contain water vapor up to saturation, and low percentages j of carbon dioxide, both of which are removed during the Vcooling process, as Will appear.

The feed gas initially is subjected to cooling by passing successively through the denoted high level and low level refrigeration stages, Which employ extraneous refrigerants desirably having boiling temperatures at least generally corresponding to those of propane and ethane. Preferably, although in recognition of equivalencies on the basis stated, the feed gas in the high level refrigeration stage is cooled by indirect exchange with propane at controlled vaporizing temperature, and in the low level 3 refrigeration stage, with ethane under controlled vaporization. It will be further understood that each refrigeration stage may employ any suitable number of exchangers, of which two in each instance are illustrative.

Entering the system, the feed gas in line flows successively through exchanger 11, separator 12, exchanger 13, separator 14, exchanger and exchanger 16. In exchanger 11 the feed undergoes indirect exchange cooling with propane supplied as liquid through line 17 and subjected to vaporization, or partial vaporization, by pressure reduction at valve 18 which may be regulated by a suitable control 19 in accordance with desired predetermined pressure, and therefore temperature, in the propane passage of the exchanger. The exchanger effluent is taken through line 20 to receiver 21, from which propane vapor is sent via line 22 to compressor 23. The latter operates to discharge the vapor through line 24 at a pressure sufficiently high that the vapor will condense to liquid propane in cooler 25, from which the condensate goes through line 26 to receiver 27, thence to be recycled through line 28 and exchanger 29 to the previously mentioned line 17.

Line 28 may also supply liquid propane by way of line 30 to the exchanger 13, all in a manner and under control similar to the previously described refrigeration cycle applying to exchanger 11. Thus parts of the system associated with exchanger 13 are given the same reference characters, with primes added, as the corresponding parts and lines associated with exchanger 11. As illustrated the line 22 propane vapor goes to compressor 23 for compression into line 24, together with the line 22 vapor.

The feed gas is subjected to dehydration in advance of l the later described alternate exchangers, by contacting the gas with liquid desiccant of suitable moisture absorptive properties, preferably though typically, ethylene glycol. The lean glycol solution is supplied from receiver 31 through line 32 to be injected through line 33 into the gas stream in line 10, here shown as being in advance of the exchanger 11. The admixed glycol and feed gas undergo cooling in the exchanger and then flow into separator 12 which is shown to have a lower pot extension 34 into which the glycol with its absorbed water separates for withdrawal under control of the liquid level regulated valve 35 for removal through line 36 to the rich glycol receiver 37. From the latter, the rich solution is taken to a suitable regenerator diagrammatically indicated at 38, wherein the solution is heated to a temperature sufficiently high to vaporize the absorbed water, leaving a lean or regenerated solution which goes, with or without cooling, to the receiver 31.

Depending upon the composition of the feed gas, the latter may or may not contain relatively heavy hydrocarbons existing in condensed state Within the separator 12, and which it is desired to remove from the system. For this purpose, the separator 12 is provided with a hydrocarbon draw-off line 39 at a higher elevation than the bottom pot glycol withdrawal, through which the lighter gravity hydrocarbons are removed under control of the liquid level regulated valve 40, to be sent through line 41 to the hydrocarbon receiver 42, from which the condensate may be taken for any desired further treatment or fractionation.

Additional glycol is shown to be introduced through line 32 at 32A into the gas stream beyond separator 12 and in advance of exchanger 13, the gas and glycol mixture again being subjected to lower temperature cooling and discharged into separator 14 from which the rich glycol solution is removed through line 44 connecting with previously described line 36, all in the manner previously described. Similarly, hydrocarbon condensate accumulating in separator 14 may be Withdrawn through line 45 and sent to receiver 42 along with the line 41 stream.

Entering the low level refrigeration stage, the line 10 gas stream flows through exchanger 15 in indirect heat exchange with ethane supplied through line 46 and pressure reduced at valve 47 to a pre-determined degree by the pressure responsive control 48. The exchanger ethane efuent passes through line 49 to receiver 50 from which the vapor is taken through line 51 to compressor 52. As fed to the exchanger 15, which mechanically may be of any suitable type as of a tube bundle design, the main gas stream will contain residual moisture, and perhaps some carbon dioxide, which require removal to a degree of substantial completeness, in advance of final flash cooling of the stream condensate. Accordingly, the ethaneproduced refrigeration in exchanger 15 will be at a temperature sufficiently 10W to cause substantially all of the moisture in the stream undergoing liquefaction to conm dense out on the exchanger surfaces, as will also most of any carbon dioxide in the gas. Normally, in the course of continued operation, the exchanger 15 would become plugged with the accumulated solids and require shut down of the system for ice removal from the exchanger.

In accordance with the invention, continuity of operation is assured by the provision of a second or alternate exchanger 54 to which the line 10 stream may be switched by way of line 55 upon taking exchanger 15 out of service by closing valves 56 and 57, valves 58 and 59 becoming opened. As before, exchanger 54 receives liquid ethane from line 46 under control of the pressure regulated valve 60, the exchanger ethane effluent going through line 61 to the receiver 50. During the on-stream operation of exchanger 54, exchanger 55 of course will be de-iced for return to service in alternation with exchanger 54.

Ethane from line 46 also feeds exchanger 16 through line 62 under control of valve 47 operating in response to the pressure responsive device 48'. Ethane vapor taken through line 49 to receiver 50 is taken through line 51 to compressor 52 to be compressed along with the line 51 gaseous ethane for delivery through line 63 and exchanger 64 to receiver 65 which supplies line 46.

Provision may be made for indirect exchange cooling of the ethane in exchanger 64, by supplying thereto through line 66 liquid propane from line 30, under regulation by valve 67 operated by the pressure responsive control 68, the exchanger propane effluent being taken through line 69 to receiver 70 and thence returned to compressor 23 by way of line 71.

Leaving the low level refrigeration stage, the line 10 stream composed' of liqueed hydrocarbons including methane, together with any uncondensibles which ordinarily would be principally nitrogen, enter separator 67 from which fixed gases together with some gaseous hydrocarbon are removed through line 68 past back pressure control valve 69 and through exchanger 70 to be heat exchanged in exchanger 29 with the propane stream therein. Beyond exchanger 29 the gaseous removal from separator 67 may be taken through line 71 to receiver 72 for use as fuel gas or any other desired disposal.

The line 10 liquefied gas, predominately methane, taken from separator 67 past liquid level controlled valve 73, is flashed by pressure reduction at the valve into separator 74 with the result that the liquid undergoes partial flash vaporization with resultant cooling according to the Joule-Thompson effect. If desired, the liquid may be subjected to one or more successive partial ash vaporizations and coolings, as by pressure reduction past valve 75 into separator 76, from which the nally cooled liquefied gas is withdrawn with or Without further pressure reduction at the liquid level controlled valve 78, to storage .79 or other disposal as into a transmission line. Vapors from separators 74 and 76 are taken through lines 80 and 81 to compressor 82. Vapors from storage 79 are taken through line 83 to compressor 84 which discharges through line 85 to the compressor 82. The latter operates to compress the mainly methane gas which is taken -through line 85 in indirect heat exchange with the separator 67 overhead, and thence is returned to line 55 to join the line 10 gas stream going to one or the other of exchangers 1S and 54. In this manner that portion of the liquefied gas stream vaporized for auto-refrigeration beyond separator 67, is retained in the System to undergo cooling and recondensation in the low level refrigeration stage.

As illustrative operating conditions, assuming the previously stated gas feed composition entering the system at about 70 F. and 520 p.s.i.a., and using propane and ethane respectively in the high level and low level refrigeration stages, the feed stream Will undergo cooling to temperatures in the order of about -12 F. through exchanger 11; -40 F. through exchanger 13; -93 F. through exchanger 1S; and will enter separator 67 beyond exchanger 16 at a temperature of about 135 F. and pressure in the neighborhood of 480 p.s.i.a. At valve 73 the stream will be llashed to a lower pressure and temperature of about 300 p.s.i.a. and 153 F. in separator 74, and at valve 75 to a pressure of about 55 p.s.i.a. and 215 F. temperature in separator 76. Beyond valve 78 the finally cooled stream will enter storage at a temperature of about 250 F. to be maintained in a typical instance at pressure slightly in excess of atmospheric pressure. As will be understood, the inherent properties of propane and ethane, or their equivalents, together with the described vaporization controls, are operative to maintain the stated temperatures in the high level and low level refrigeration stages, and similarly by reason of its inherent properties, the condensate beyond separator 67 may be controllably flashed to produce the iinal stage or incremental coolings.

We claim:

1. The method of liquefying natural gas under pressure and containing predominately methane together With water vapor and containing also hydrocarbons higher boiling than methane, that includes first subjecting the stream to rst stage coooling by indirect heat exchange with propane undergoing vaporization and then to second stage cooling by indirect heat exchange with ethane undergoing vaporization, introducing separate streams of glycol desiccant into the gas stream for partial moisture absorption therefrom in the lirst of said stages, removing the glycol together with liquefied hydrocarbons and separating the hydrocarbons from the -removed glycol, regenerating and returning the glycol to the gas stream in said separate streams, maintaining separate cooling exchangers, passing said gas stream after said glycol and moisture removal alternately through said exchangers to alternately freeze out residual moisture on cooling surfaces of the exchangers and to cool the gas stream to a temperature at which it is at least partially liquefied, separating resulting liquid from gas contained in the stream, then expanding and partially vaporizing the liquefied gas to further cool the liquid, storing the residual cooled liquid, and compressing and returning to the stream undergoing cooling gas resulting from said partial vaporization.

2. The process of claim 1, in which the natural gas contains nitrogen which remains gaseous after passage through said exchangers and which is separated from the yliquefied gas in advance of said partial vaporization thereof.

3. The method of liquefying condensable hydrocarboncontaining natural gas under pressure and containing predominately methane together with water vapor, that includes subjecting a stream of the gas to successive stages of coolings to lower temperatures, removing a portion of the moisture from the stream at an early cooling stage by introducing liquid desiccant to the stream to absorb moisture, separating the water-containing desiccant from the stream and regenerating and returning the lean desiccant thereto, maintaining separate cooling exchangers, passing said stream after said moisture removal alternately through said exchangers to alternately freeze out residual moisture on cooling surfaces of the exchangers and to cool the stream to a temperature at which it is at least partially liqueed, separating resulting liquid from gas contained in the stream, then expanding and partially vaporizing the liquefied gas to further cool the liquid, and storing the lresidual cooled liquid, hydrocarbons contained in the gas being condensed and removed from the gas stream both prior to and following its passage through one of said exchangers.

4. The method of claim 3, in which the natural gas contains nitrogen, a portion of which remains gaseous after passage through said exchangers and which is separated from the liquefied gas in advance of said partial vaporization thereof, gas resulting from said partial vaporization being compressed and returned to the stream undergoing cooling.

5. The method of liquefying condensable hydrocarboncontaining natural gas under pressure and containing predominately methane together with water vapor, that includes subjecting a stream of the gas to successive stages of coolings to lower temperatures, removing a portion of the moisture from the stream at an early cooling stage by introducing ethylene glycol to the stream to absorb moisture, separating the water-containing glycol from the stream and regenerating and returning thelean glycol thereto, maintaining separate cooling exchangers, passing said stream after said moisture removal alternately through said exchangers to alternately freeze out residual moisture on cooling surfaces of the exchangers and to cool the stream to a temperature at which it is at least partially liqueed, separating resulting liquid from gas contained in the stream, then expanding and partially vaporizing the liqueed gas to further cool the liquid, and storing the residual cooled liquid.

6. The method of liquefying condensable hydrocarboncontaining natural gas under pressure and containing predominately methane together with water vapor, that includes subjecting a stream of the gas to successive stages of coolings to lower temperatures, removing a portion of the moisture from the stream at an early cooling stage, maintaining separate cooling exchangers, passing said stream after said moisture `removal alternately through said exchangers to alternately freeze out residual moisture on cooling surfaces of the exchangers and to cool the stream to a temperature at which 'it is at yleast partially liquefied, separating resulting liquid from gas contained in the stream, then expanding and partially vaporizing the liquefied gas to further cool the liquid, compressing gas resulting from said partial vaporization and returning the compressed gas to the stream undergoing cooling, and storing the residual cooled liquid.

References Cited by the Examiner UNITED STATES PATENTS 2,151,248 3/ 1939 Vaughan 62-20 X 2,509,034 5/ 1950 Claitor 62-40 X 2,584,985 2/ 1952 Cicalese 62-13 2,617,275 11/1952 Golf 62-14 2,622,416 12/ 1952 Ogorzaly 62-14 2,643,527 6/ 1953 Keith 62-13 2,716,332 8./ 1955 Haynes 62-20 2,758,665 8/ 1956 Francis 62-20 2,765,637 10/ 1956 Etienne 62-29 2,801,207 7/ 1957 Laurence.

2,8 12,646 11/ 1957 Twomey.

2,960,837 11/ 1960 Swenson 62-40 3,020,723 2/ 1962 De Lury 62-40 X NORMAN YUDKOFF, Primary Examiner. J. JOHNSON, Assistant Examiner. 

6. THE METHOD OF LIQUEFYING CONDENSABLE HYDROCARBONCONTAINING NATURAL GAS UNDER PRESSURE AND CONTAINING PREDOMINATELY METHANE TOGETHER WITH WATER VAPOR, THAT INCLUDES SUBJECTING A STREAM OF THE GAS TO SUCCESSIVE STAGES OF COOLINGS TO LOWER TEMPERATURES, REMOVING A PORTION OF THE MOISTURE FROM THE STREAM AT AN EARLY COOLING STAGE, MAINTAINING SEPARATE COOLING EXCHANGERS, PASSING SAID STREAM AFTER SAID MOISTURE REMOVAL ALTERNATELY THROUGH SAID EXCHANGERS TO ALTERNATELY FREEZE OUT RESIDUAL MOISTURE ON COOLING SURFACES OF THE EXCHANGERS AND TO COOL THE STREAM TO A TEMPERATURE AT WHICH IT IS AT LEAST PARTIALLY LIQUEFIED, SEPARATING RESULTING LIQUID FROM GAS CONTAINED IN THE STREAM, THEN EXPANDING AND PARTIALLY VAPORIZING THE LIQUEFIED GAS TO FURTHER COOL THE LIQUID, COMPRESSING GAS RESULTING FROM SAID PARTIAL VAPORIZATION AND RETURNING THE COMPRESSED GAS TO THE STREAM UNDERGOING COOLING, AND STORING THE RESIDUAL COOLED LIQUID. 